1. Field of the Invention
The present invention relates to a servo writer and a servo writing method for writing a servo signal onto a servo band of a magnetic tape.
2. Description of the Related Art
In recent years, high density recording design in magnetic tapes has advanced, and some magnetic tapes for backup mediums of computers have a recording capacity of several hundreds of gigabytes. Magnetic tapes therefore have several hundreds of data tracks along the width thereof. Such high density recording design involves narrowing of the data tracks and of the intervals between the adjacent data tracks in a magnetic tape. Thus, in order to allow recording/reproducing devices of a magnetic head to trace such narrow data tracks, servo signals are written onto a magnetic tape in advance, and the servo signals are then read by the magnetic head, while the position of the magnetic head relative to the magnetic tape (the position along the width of the magnetic tape) is servo-controlled (refer to Japanese Unexamined Patent Application (KOKAI) Heisei No. 8-30942 (Paragraph No. 0016 and FIG. 1)).
The above servo signals are written onto the servo bands of a magnetic tape with a servo writer, by applying a recording current to a magnetic head of the servo writer so as to magnetize the non-magnetized servo bands in one direction. Conventionally, as shown in FIG. 5A, the servo signals SS are recorded onto the corresponding non-magnetized servo bands SB, by feeding, to a magnetic head, a recording pulse current PC constituted of a zero current ZC and a plus pulse current PP, as a recording current. In the recording pulse current PC, as shown in FIG. 5B, when the zero current ZC out of the recording pulse current PC is fed to the magnetic head, the servo bands SP are not magnetized. On the other hand, when the plus pulse current PP is fed thereto, due to leakage flux generated from the head gaps of the magnetic head, the servo bands SB are magnetized in one direction to form servo patterns SP, so that the servo signals SS are written onto a magnetic tape MT. Here, each space between the adjacent servo bands SB serves as a data band DB onto which data signals are to be written.
Each servo signal SS is formed by repeating, at predetermined intervals in the length of the magnetic tape MT, a servo pattern SP including a burst Ba and a burst Bb. Incidentally, the burst Ba is a portion which is constituted of two magnetized stripes having a slope of positive angles relative to the traveling (transport) direction, and the burst Bb is a portion which follows the burst Ba and which is constituted of two magnetized stripes having a slope of negative angles relative to the traveling direction. Here, each servo pattern SP is constituted of four stripes having slopes of positive and negative angles in twos, but modifications can be made as appropriate. For example, it may be constituted of ten stripes having slopes of positive and negative angles in fives and, further two types of servo patters SP may be alternately arranged; one is constituted of ten stripes having slopes of positive and negative angles in fives and the other is constituted of eight stripes in fours. In FIG. 5B, the servo patterns SP are enlarged relative to the magnetic tape MT, for the sake of understanding.
The magnetic tape recording/reproducing device has at least one servo signal read device, such as MR element which changes its electrical resistance in accordance with a magnetic quantity. The magnetic tape recording/reproducing device detects, based on the variation of electrical resistance of the above MR element, the variation of the magnetic field generated from the servo signal SS, and outputs the variation of the magnetic field as a read signal in a differential waveform mode (voltage value). As the variation of electrical resistance of the MR element is increased, the peak voltage value obtained by reading the servo signal SS is increased. As a result, the output read from the servo signal SS has an improved S/N ratio. Accordingly, when the magnetic field generated from the servo signal SS has a great variation, or when the servo signal read device (MR element) has a large sensing area due to its wide dimensions, the output RSL read from the servo signal SS is increased as shown in FIG. 5(c).
In future, it is expected that magnetic tapes will advance to have a memory capacity of several tens of terabytes. As such high density recording design proceeds, the number of data tracks formed on a magnetic tape is increased, the width of data tracks and interval between adjacent data tracks are further narrowed, and a magnetic tape itself is thinned. Accompanied with this, magnetic quantity which can be detected from the servo signals SS on a magnetic tape is decreased. Further, the variation of the magnetic quantity which can be detected from the servo signals SS by the servo signal read device of a magnetic head is also decreased. Due to this decrease, the output RSS read from the servo signal SS has a low peak voltage value as shown in FIG. 5D, in other words, the S/N ratio of the output RSS is deteriorated. Finally, the magnetic tape recording/reproducing device cannot read the servo signals SS correctly, whereby the position of the magnetic head is unable to be controlled with high precision.
In view of the above problem, the applicant has previously discovered that the servo bands SB of a magnetic tape MT are magnetized by use of a DC erase head (not shown) in one direction (i.e., DC magnetization) along the length of the magnetic tape MT, and the servo signals SS are then recorded onto the servo bands SB by magnetizing them in the direction opposite to the one direction (refer to FIG. 6A). Incidentally, in FIG. 6A, the magnetized directions are denoted by small arrows. The output (peak voltage value) which is read from the servo signal SS by the servo signal read device depends on a variation ratio or amount at a boundary between portions onto which a signal is not recorded and onto which a signal is recorded. For this reason, an orientation of a magnetic field greatly changes from the forward to reverse directions at the boundary between the forwardly magnetized portion and the reversely magnetized portion on each servo band. Similarly, the orientation of a magnetic field greatly changes from the reverse to forward directions at the boundary between the reversely magnetized portion and the forwardly magnetized portion. This makes it possible to obtain the large output from the servo signal SS, as shown in FIG. 6B. In other words, it is possible to improve the S/N ratio of the output.
In the above technique, however, a constant current is fed to a DC erase head and a servo signal write head in the servo writer. This causes a problem that, when the servo signal is written by the single servo writer onto a servo band on different thin magnetic layers of magnetic tapes, the reproducing characteristics of the servo signal differ depending on the thicknesses of the magnetic layers. For example, there is a possibility that the reproduction of the servo signal exhibits excellent characteristics against thin magnetic tapes, whereas it exhibits faulty characteristics against thick tapes.